Logical Database Design for the Relational Model Step

Logical Database Design for the Relational Model

Step 2 Build and Validate Logical Data Model u To translate the conceptual data model into a logical data model and then to validate this model to check that it is structurally correct using normalization and supports the required transactions. 2

Build and Validate Logical Data Model u Derive relations for logical data model – To create relations for the logical data model to represent the entities, relationships, and attributes that have been identified. 3

Conceptual data model for Staff view showing all attributes 4

Derive relations for logical data model (some examples) u One-to-many (1: *) binary relationship types – For each 1: * binary relationship, the entity on the ‘one side’ of the relationship is designated as the parent entity and the entity on the ‘many side’ is designated as the child entity. To represent this relationship, post a copy of the primary key attribute(s) of parent entity into the relation representing the child entity, to act as a foreign key. – Ex. Staff Registers Client – Staff (Staff. No = primary key) 5 – Client(Staff. No = foreign key)

Step 2. 1 Derive relations for logical data model u Superclass/subclass relationship types – Identify superclass entity as parent entity and subclass entity as the child entity. There are various options on how to represent such a relationship as one or more relations. – The selection of the most appropriate option is dependent on a number of factors such as the disjointness and participation constraints on the superclass/subclass relationship, whether the subclasses are involved in distinct relationships, and the number of participants in the superclass/subclass relationship. 6

Guidelines for representation of superclass / subclass relationship 7

Example u Owner – superclass (see next slide) u We would select option 3 - disjoint and mandatory u If it had been nondisjoint (option 1) – then we would need discriminators p. Owner. Flag and b. Owner. Flag – indicating whether each row belongs to Private. Owner or Business. Owner. 8

Conceptual data model for Staff view showing all attributes 9

Representation of superclass / subclass relationship based on participation and disjointness 10

Example owne r. No Addr tel. No f. Nam ess e l. Nam e O 11 23 st 123 smith O 12 55 st 456 O 13 66 st 789 jane John smith b. Name b. Type contract Name p. Owne b. Own r. Flag er. Flag 1 Neiu institution contract 1 1 Old navy retail 1 contract 2 1 11

Representation of superclass / subclass relationship based on participation and disjointness, cont. u Different Options: – Option 1: 2 different flags (like below) each indicating whether the row is a member of a particular subclass (since it could be a member of one or more. – Option 2: 1 flag that distinguishes whether the row is a member of Private. Owner, Business. Owner, or both. – Option 3: No flags – just test whether the attributes have values. If null then it’s not a member of that class. 12

Validate relations using normalization u Next validate the relations in the logical data model using normalization. u Review the logical data model with the users to ensure that they consider the model to be a true representation of the data requirements of the enterprise. 13